1. Field of the Invention
This invention relates to a laminated glazing pane for transportation vehicles. Particularly, the laminated glazing pane may be used as an aircraft windshield.
2. Discussion of the Background
Laminated glazing panes for use as an aircraft windshield are generally comprised of two thermally or chemically toughened glass sheets and a thermoplastic intermediate film composed of several plies of plasticized polyvinyl butyral (hereinafter referred to as PVB). Plasticized PVB is selected for this application because of its high modulus in tension and high elongation at rupture, and because it can absorb energy while assuring leak-tightness of a pane in which all the glass sheets are broken.
When a laminated glazing pane of this type is used in an aircraft at high altitude, it is subjected to considerable pressure and temperature differences on its two faces. Because the coefficient of expansion of PVB is larger than the coefficient of expansion of the glass sheets, the pane is subjected to stresses, such as shear stresses, at the edges of the pane. These stresses are further increased when the pane is subjected to shocks or impacts from, for example, a bird striking the windshield. Furthermore, the modulus of tension of PVB is increased at low temperatures such as below 0.degree. C.
These stress problems are further accentuated when the intermediate film comprises, embedded in its edges, a peripheral band of a metal or a laminated material, which increases the rigidity of the pane and/or makes mounting by bolting, for instance, of the pane into the body work of the aircraft cockpit easier.
Proposed solutions to the above described stress problems are directed at reducing the stresses referred to above, thereby reducing the risk of failure of the pane by reducing the stresses in the pane. One of the proposed solutions consists of applying a material acting as a separator into the marginal portion of the pane. This solution may be suitable for panes of small dimensions. However, it is unsatisfactory for panes the size of an aircraft windshield. Furthermore, the separator materials may promote the penetration of humidity into the intervening PVB, leading to loss of bond at undesirable locations, as well as to loss of optical quality of the pane such as transparency. In addition, separator materials not having the optical quality of the intermediate films used in laminated panes reduce the field of view of the pane.
French Patent Publication FR-A-2 310 979 suggests placing continuous polyurethane film between the PVB film and the glass sheet. The polyurethane film deflects the shear forces and thus reduces the effect of the thermal contraction of the PVB. However, this proposed solution generally requires an activation of the bond for the polyurethane over the entire contact surface with-the glass. As a result, a supplementary interface between two different thermoplastic products having different indices of refraction is created which can introduce optical defects into the pane.